Atomic force microscopy (AFM) involves monitoring the displacement of a sharp probe tip as it is scanned over a sample. The probe tip is usually attached to a cantilever and displacement of the probe tip can be monitored using many different techniques. One widely-used technique for monitoring displacement of the probe tip involves reflecting a laser beam off the cantilever and detecting angular deflection of the reflected laser beam using a position-sensitive detector having two adjacent photodiodes. Angular deflection of the cantilever causes a two-fold larger angular deflection of the laser beam and the difference between the output signals of the two photodiodes indicates the position of the laser beam. The position of the laser beam relative to the two adjacent photodiodes indicates the angular deflection of the cantilever and thus the displacement of the probe tip.
Although a displacement monitoring technique that relies on deflection of a laser beam is quite sensitive, the sensitivity of the technique is limited by the maximum laser power that can be conveyed to the detector and the diffraction limit set by the cross-sectional size of the laser beam that is incident on the cantilever and the position-sensitive detector.